Question 944220
you can use the rule of thumb for normal distribution to figure this out.
the rule is:


68% are within 1 standard deviation from the mean.
95% are within 2 standard deviations from the mean.


the difference between 95 and 68 are the percent that are between 1 and 2 standard deviations from the mean plus or minus.


the difference between 100% and 95% are the percent that are greater than 2 standard deviations from the mean plus or minus.


you will get:


68% are within 1 standard deviation from the mean.


95% - 68% = 27% are between 1 standard deviation and 2 standard deviations from the mean.


100% - 95% = 5% are beyond 2 standard deviations from the mean.


Since A and B are above the mean and D and F are below the mean, and since the distribution curve is symmetric, you will get:


68% will get a C because they are within 1 standard from the mean.  Half of them will be above the mean and half of them will be below the mean.


13.5% will get a B because they are between 1 and 2 standard deviations above the mean.


13.5% will get a D because they are between 1 and 2 standard deviations below the mean.


2.5% will get an A because they are beyond 2 standard deviations above the mean.


2.5% will get an F because they are beyond 2 standard deviations below the mean.


Add up all the percentages and you get 68 + 2 * 13.5 + 2 * 2.5 = 100%.